The present disclosure generally relates to downhole tools for use in oil and gas wellbores and, more particularly, to the downhole transport and setting of wellbore isolation devices such as downhole bridge plugs and frac plugs.
In the drilling, completion, and stimulation of hydrocarbon-producing wells, a variety of downhole tools are used. For example, it is often desirable to seal portions of a casing string extended within the wellbore, such as during fracturing operations when various fluids and slurries are pumped from the surface into the casing string and forced out into a surrounding subterranean formation. It thus becomes necessary to seal the casing and thereby provide zonal isolation. Wellbore isolation devices, such as packers and bridge plugs, are designed for these general purposes and are well known in the art of producing hydrocarbons, such as oil and gas.
Another type of wellbore isolation device is a fracturing plug or “frac” plug, which is essentially a downhole packer with a ball seat for receiving a frac ball. When the frac plug is set and the frac ball engages the ball seat, the casing string or other tubing in which the frac plug is set is effectively sealed and fluids flowing from the surface are thereby prevented from bypassing the frac plug. At this point, a “fracking” fluid or slurry can be pumped into the well and is thereby forced into the surrounding subterranean formation above the frac plug.
Frac plugs are typically attached to a conveyance at the surface and pumped to a target zone within the well using hydraulic pressure applied from the surface. Pump down operations in horizontal wells, however, are often frustrated when there is sand, wellbore debris, or other downhole obstructions built up or otherwise disposed within the well or casing string. When the frac plug reaches such downhole obstructions it tends to either slow down or come to a complete stop altogether.
Slowing or stopping the frac plug results in the build up of hydraulic pressure behind the frac plug within the casing string. In some cases, the increased hydraulic pressure forces or impels the frac plug through the downhole obstruction. In such cases, the full force of the surface pump is assumed by the conveyance which can result in the frac plug or other attendant downhole tools being severed from or “pumped off” the conveyance. It would be advantageous to have a device or system capable of automatically relieving downward hydraulic pressure on the frac plug under such conditions, and thereby prevent unwanted frac plug pump offs.